1. Field of the Invention
The present invention relates to a process for preparing hydroxylated aromatics by using hydrogen and oxygen and more particularly, to a process for preparing hydroxylated aromatics by using hydrogen and oxygen with a two-component heterogeneous catalyst. One component consists of porous catalyst containing one of Group VIII B transition metals such as Pd, Pt, Au and Cu, and hydrogen transfer organic compounds such as anthraquinone. The other component consists of a catalyst containing a transition metal selected from Ti, V, and Sn with tetrahedral coordination geometry. The main advantages of this new catalytic system are to 1) overcome the drawbacks of liquid phase oxidation using conventional homogeneous catalysts, 2) avoid use of expensive hydrogen peroxide as an oxidant, and 3) improve the selectivity of the reaction.
2. Description of the Related Art
Molecular oxygen (O.sub.2), organic peroxide, and hydrogen peroxide have been used, in general, as selective oxidants for liquid phase oxidation of organic compounds.
A process for oxidation of benzene to phenol by using molecular oxygen as an oxidant has been disclosed. For example, benzene conversion rate is 15% and the selectivity towards phenol is approximately 70% in the presence of palladium supported on heteropolyacid catalyst in a pressure reactor at a temperature of 130.degree. C. under 60 atmosphere of oxygen for 4 hours [J. Mol. Catal. A., 1977, 120, 117]. To produce commercially available phenol, this reaction requires introduction of oxygen at high pressure which is dangerous to justify commercialization.
The process where organic peroxide such as cumen is used as an oxidant has been widely employed. But the process is uneconomical due to multi-steps of reaction and production of by-products such as acetone which causes environmental problem.
Due to above mentioned problems, hydrogen peroxide is preferred as an oxidant in the oxidation of organic compounds than molecular oxygen and organic peroxide. Hydrogen peroxide is very selective for the oxidation and decomposes to oxygen and water which are environmentally acceptable.
Processes for the introduction of a hydroxyl group onto the aromatic compounds have been reported. Followings are some examples.
A process for hydroxylation of aromatic hydrocarbons in the presence of Fe(II), ascorbic acid and EDTA at room temperature is disclosed. But it is not recommendable to apply this reaction for commercial exploitation due to extremely low yield [J. Biol. Chem., 1954, 208, 731].
Another process for benzene hydroxylation to phenol is disclosed in which Fenton reagent comprising Fe(II) and hydrogen peroxide are used [J. Am. Chem. Soc., 1975, 97, 363], therein active substance of Fenton reagent is hydroxyl radical derived from the reaction.
Another process for the hydroxylation of aromatic hydrocarbons by molecular oxygen is disclosed in the presence of metal ion such as Cu(I), Sn(II), Ti(III) or Fe(II)-EDTA of which standard oxidation/reduction electrode potential (E.sub.0) is 0.15V.
In addition, a Cu--Pd incorporated silica catalyst has been known to be effective in the hydroxylation of benzene [Catal. Lett., 1990, 4, 139]. In the process, reaction products are phenol and hydroquinone. In this reaction, Cu(I) reacts with oxygen in acidic condition to form hydrogen peroxide, which is reduced to hydroxyl radical to convert benzene to phenol in the reaction mixture. Cu(II), here, should be reduced to Cu(I) for this catalytic reaction, palladium-incorporated silica catalyst can be used to permit hydrogen reduction.
New type of molecular catalysts which incorporate titanium in the molecular framework such as titanium silicate (TS-1 zeolite) have been recently developed for the hydroxylation of aromatic compounds such as benzene and saturated hydrocarbons in the presence of hydrogen peroxide under mild conditions [Appl. Catal. 1990, 57, L1; J. Mol. Catal., 1991, 68, 45]. TS-1 zeolite is a material with the structural features of ZSM-5 in which content of silicone is considerably high and Ti(IV) is coordinated as tetrahedral geometry in the framework at a low concentration of 0.1 to 2.5 mole %. Low content of titanium contributes to high dispersion of catalytic active site, resulting in the increase of oxidation selectivity. Reaction which employs TS-1 zeolite can be performed in 30% hydrogen peroxide aqueous solution under mild condition at a temperature of 20.degree. C. to 100.degree. C.
Because microporous zeolites have limitation for diffusion of large molecules, there is a need to develop and apply mesopore zeolite based catalyst that have larger pores.
Mesoporous Ti-MCM-41 shows lower conversion rate in the hydroxylation of small molecules such as 1-hexane when compared to the activity of TS-1 or Ti-.beta.. However, Ti-MCM-41 shows higher selectivity than Ti-.beta. in the oxidation of .alpha.-terpineol and norbornene. These results suggest that mesopore molecular sieves could be used for oxidation of large molecules.
Similar to the present invention, a two-component catalyst (Pd/C+TS-1) containing palladium and titanium has been applied for the hydroxylation of benzene with hydrogen and oxygen at atmospheric pressure at 35 .degree. C. [J. Chem. Soc. Chem. commun. 1992, 1446], but this reaction has drawback of low reactivity.
A silica catalyst containing both copper and palladium has also been utilized to oxidize benzene with hydrogen and oxygen under atmospheric pressure and at a temperature of 50.degree. C. [JCS Perkin Trans. 2, 1990, 1991], but this reaction has the maximum turnover number of about 3.8.
As mentioned above, direct oxidation of benzene to phenol by hydrogen peroxide has numerous problems which include the following: (1) high costs of hydrogen peroxide; (2) low stability due to the impurities derived from the process of preparation of hydrogen peroxide; (3) difficulties in storage and transport. Therefore, it can be suggested that an oxidation process that utilizes reaction with hydrogen and oxygen, instead of hydrogen peroxide, and conducted at mild reaction conditions would be more environmentally acceptable.